Nicolas Thébaud

Archean komatiite volcanism controlled by the evolution of early continents

Significance Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very rarely since. They are the signature rock type of a hotter early Earth. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. This study used a combination of different radiogenic isotopes to map the boundaries of Archean microcontinents in space and time, identifying the microplates that form the building blocks of Precambrian cratons. Isotopic mapping demonstrates that the major komatiite belts are located along these crustal boundaries. Subsequently, the evolution of the early continents controlled the location and extent of major volcanic events, crustal heat flow, and major ore deposit provinces. The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earths continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Spatio-temporal constraints on lithospheric development in the southwest–central Yilgarn Craton, Western Australia

The Archean western Yilgarn Craton contains an extensive record of supracrustal formation from ca 3730 to ca 2675 Ma, as well as evidence of an ensialic crustal component as old as ca 4400 Ma. These features make the western Yilgarn Craton one of the oldest crustal provinces on Earth and ideal for the study of Archean crustal evolution. Spatial analysis of new and collated U–Pb age data define three broad pulses of granite emplacement at ca 3000–2820, ca 2805–2720 and ca 2720–2600 Ma, with a period of regional quiescence at 2820–2805 Ma. Within these pulses, major peaks in granite production are defined at ca 2920, ca 2890, ca 2845, ca 2790, ca 2750, ca 2690, ca 2665, ca 2655, ca 2630, and ca 2615 Ma; with lesser inherited material as old as 3670 Ma. In the western Yilgarn Craton, all terranes show evidence of granite activity at ca 3000–2820 Ma. The South West Terrane and Southern Cross Domain share granite pulses at ca 2950–2920, 2880–2820 and 2800–2720 Ma, although during these intervals granite magmatism tends to dominate in one terrane, i.e. ca 2805–2780 Ma granite activity predominantly occurs in the South West Terrane, while 2780–2720 Ma activity is focused in the Southern Cross Domain. Including the period of quiescence, granite production is relatively minor between ca 2820 and ca 2720 Ma relative to the 3000–2820 Ma and 2720–2600 Ma intervals, suggesting limited crustal development at this time. This period corresponds with widespread greenstone formation throughout the western Yilgarn Craton. The major pulse of granite emplacement and crustal evolution occurs at ca 2700–2600 Ma, with the main phases of activity at ca 2680–2650 Ma in the Southern Cross Domain and ca 2640–2620 Ma in the South West Terrane. These pulses coincide with a craton-wide transition in granite geochemistry from high-Ca to low-Ca at ca 2650 Ma and suggest significant variations in the method and timing of melt generation. Results from this study provide new constraints on the spatio-temporal evolution of the lithosphere in the western Yilgarn Craton. The spatial distribution of these age data suggest that existing terrane boundaries should be revised with the South West Terrane separated into at least two distinct domains, and the boundary between the Youanmi and South West Terranes moved westward to correspond with the eastern extent of charnockite granites.

New insights into Archean granite-greenstone architecture through joint gravity and magnetic inversion

Archean tectonic evolution and metallogenic endowment in cratonic areas are intrinsically linked to lithological diversity and complex architectures. Unfortunately, reliable models of the deep structure of such areas are scarce due to poor surface exposure and limited availability of drill holes and high-resolution geophysical information. By combining multiple geophysical data sets, the strategy of cross-gradient joint geophysical inversion permits improved subsurface imaging of geological heterogeneity and structure. In this work we showcase the application of cross-gradient joint inversion of gravity and magnetic data to characterize the heterogeneity of the Leonora district in the eastern Yilgarn Craton, Western Australia. Our results harmonize both geophysical data types into integrated images of the top 15 km of the crust and successfully predict observations made by a seismic experiment and surface geological mapping.

Geochemistry of komatiites in the Southern Cross Belt, Youamni Terrane, Western Australia

The Southern Cross belt is located in the Youanmi Terrane of the Yilgarn Craton, directly north of the Forrestania belt, which contains economic komatiite-hosted Ni–Cu mineralisation. The mafic and ultramafic greenstone assemblage in the Southern Cross belt, encompasses a continuum from olivine-rich cumulates through to evolved high MgO komatiitic basalts and gabbros. Inspection of geochemical data shows that komatiites can be divided into a high-Al group with Al2O3/TiO2 17–19, slightly lower than typical Munro-type komatiites, and a low-Al group with Al2O3/TiO2 17–19, intermediate between typical Munro-type and Barberton-type komatiites. Parent magmas for the high-Al and low-Al groups had MgO contents ranging between 10 and 15 wt% and between 15 and 25wt%, respectively. Volcanic facies show a transition from distal thin flow facies in the north, changing to lava lakes and sills further south. Olivine-rich channel or conduit facies assemblages are absent from both suites, suggesting low prospectivity for nickel sulfide mineralisation. The high-Al group displays flat to very weakly LREE enriched patterns with flat HREE that are typical of Munro-type komatiites. The low-Al group is weakly LREE enriched, has distinctly negative sloping REE patterns from La to Lu, and minor HREE depletion relative to MREE. Direct comparison with the Forrestania belt shows that the Southern Cross komatiites overlap with the transitional Munro-type komatiites of upper Central and Takashi belts that represent the unmineralised portion of the Forrestania belt. The predominance of Munro-type komatiites in the Southern Cross belt implies that the Southern Cross komatiites were derived from shallower melting than the Barberton-type komatiites that host mineralisation in Forrestania greenstone belt. Furthermore, the variation between Al/Ti and V/Ti implies that the Southern Cross ultramafic suite may represent the transition from Barberton-type to Munro-type komatiites, and formed close to thelower pressure limit of stability of majorite garnet in the mantle residue. It is suggested that the transitional character of the Southern Cross komatiites may be associated with a fundamental difference in the crustal architecture and craton keel thickness underlying the southeastern Youanmi Craton.

Geochronological constraints on nickel metallogeny in the Lake Johnston belt, Southern Cross Domain

Geochronology and stratigraphic revision of the Lake Johnston greenstone belt and adjacent granitoids and granitic gneiss provide new insight into the age of komatiites in the Southern Cross Domain of the Archean Yilgarn Craton. Roundtop Komatiites are geochemically similar to undated komatiites in the adjacent Ravensthorpe and Southern Cross—Forrestania greenstone belts, and the results can be extrapolated to improve the regional understanding and geodynamic evolution. Consequently, the further refined knowledge of the regional stratigraphy improves the understanding of the evolution and targeting of komatiite-hosted nickel deposits. A minimum age of ca 2773 Ma for the succession of the Lake Johnston greenstone belt is provided by crosscutting granitic rocks, with a maximum age for the underlying Roundtop Komatiite given by a maximum depositional age of ca 2876 Ma for felsic volcaniclastic rocks of the underlying Honman Formation. These new results suggest that komatiites of the Southern Cross Domain are significantly younger than previously assumed, which has implications for Yilgarn-wide geodynamic models regarding ‘plume activity’ and global correlations in the Meso- to Neoarchean.